The broad objectivesof the investigations described were aimed to examine the underlying state of host defense mechanisms in subjects with sudden unexplained death syndrome (SIDS) in infancy. Specific studies were proposed to examine, the characteristics of serumand secretory immune response to infection with common respiratory viruses, the presence and localization of infectious of fluorescent viralantigen in the respiratory tract and, in the innate state of secretory IgA immunoglobulin reactivity in the bronchopulmonary epithelium and other mucosal sites. The specific studies carried out during the current budget period focused on 1) the nature of immunologic reactivty in the respiratory tract and other mucosal surfaces in SIDS, 2) the possible relationshipof infections with respiratory syncytial virus (RSV) and other respiratory viruses to the development of SIDS, 3) the spectrum of antibody and cell mediated immune reactivity in different clinical forms by RSV infection in infancy and its possible implications in SIDS, and 4) the interaction of mucosal immunity and respiratory viral infections in an experimental animal model.